Unsure what triggered it, but these are coming at odd times and usually occurs when copying data into Gluster with a normal cp or rsync operation.
Unsure if this is supposed to be error, we should be alarmed or its just a benign message which lingers at wrong log level.

Do you see any errors for the apps? Sometimes, the kernel keeps file handles around from previous mounts or mounts that used the same server but a different export.
I've seen these errors for background apps like gnome file monitoring that try to locate a .Trash directory on the mount point.
This is not a problem if your applications are not receiving "Stale file handle".